Activity Monitor is the program that allows you to see the list of all running applications (ie., activities) and monitor which applications are consuming more CPU and which ones are consuming more memory.
Activity monitor application is located in the following path.
/Applications/Utilities/Activity Monitor.app/Contents/MacOS/Activity Monitor
However, you can always invoke this application, by pressing and holding "Command" key and hitting "SpaceBar", then typing in the words "Activity Monitor" in full or the first few characters. A list of matching application will be displayed and we should just click the one we need, as shown in the picture below.
Alternatively, if you are using Mavericks, you can just click on the Battery Icon. This action will show you the top most application that is draining the battery the most. You can launch Activity Monitor by selecting (clicking) the displayed application, as shown below.
Once you launch Activity Monitor, you can find the high-impact battery draining application, and you can also see which applications demand high performance GPU and which are being AppNapped.
Activity Monitor is very useful for application developers to see how the application is behaving under various scenarios.